The present invention relates to a method for powder coating and a plant for carrying out the method.
Powder coating is a well known method for coating of objects in which one starts with a powder coating material which is electrically charged and sprayed against the surfaces of an object, and which material is finally adhered and converted to a solid state by heating to its melting temperature. Since the powder consists of a plastic which is cured by heating, it must be heated to a comparatively high temperature, about 200xc2x0 C.
This coating method may be performed on objects having good heat resistance and a conductive surface. However, where the surface of an object is non-conductive, implying that the object cannot be grounded or supplied with a charge of an opposite polarity to the charge of the powder, difficulties arise with getting the powder to adhere to the surface of the object during the time between spraying and heating the powder to the melting temperature.
The difficulty of obtaining a polarity difference between the powder and the object, when non-conductive surfaces are involved, has in certain processes been addressed by either varnishing the object with a conductive varnish, or subjecting it to water so as to form a conductive moisture layer on the surface. These methods are, however, of limited use because of disadvantages including the additional operation and material required by varnishing, and inferior adhesion as compared to powder coating on a clean surface. Additionally, when utilizing such methods, discoloration may occur with clear varnishes.
The addition of water may impair the adhesion of the powder coating and can damage the object by confining the added water beneath the coating.
A further method of getting the powder to adhere to the surface of a non-conductive object is disclosed in Albers, German Patent No. 3,211,282. Albers teaches heating glass objects having good heat resistance to a temperature of 400-900xc2x0 C. This causes the powder granules which impact the object to melt and stick to the surface, making it possible to bring the conversion to a homogenous, solid state to an end. Objects that are heat sensitive may deform when exposed to high temperatures and cannot be treated at the high temperature required by this method. Thus, the method of Albers cannot be applied to heat sensitive objects such as wood or plastic.
An object of the present invention is thus to provide an apparatus and method for polymeric powder coating of heat sensitive objects.
In accordance with the present invention, these and other objects have now been realized by the invention of an apparatus and method for applying polymeric powder to heat-sensitive objects. The temperatures may be limited to approximately 100xc2x0 C. and below. When objects having a non-conductive surface are to be coated, the method may be carried out without varnishing with a conductive varnish or addition of moisture. The method of the present invention is therefore suitable for coating objects made of wood or wood based materials as fibre board, and objects made of a plastic, which may be chosen for reasons of tenacity or cost from a type providing the finished object with a surface having a different look than the one possible with the construction plastic itself. When wooden objects are concerned the coating may be a clear varnish, which allows the structure of the wood to stand out.
According to the invention the method comprises the following main steps:
First, preparing a powder for the coating, the powder having a low melting point of approximately 60-100xc2x0 C. and consisting of a polymer that may be cured by electromagnetic radiation, and especially radiation by ultraviolet light, at a temperature in the region mentioned above or lower.
Second, preparing the object so that the powder may be retained on its surface until a permanent adherence has been achieved by melting and curing the powder. This may be achieved in one or more of the following ways, depending upon the material and the design of the object:
Pre-heating the object to the melting temperature of the powder, making the powder granules stick to the surface during melting. This may be carried out whether or not the object has a conductive surface and with the herein disclosed powder composition at a low temperature.
Retention of the powder by means of electrostatic forces, thus giving the powder an electric potential and the object a potential of the opposite polarity. This may be achieved when objects having a conductive surface are concerned.
By applying retention by means of electrostatic forces on objects having a non-conductive surface, which thereby has to be made conductive by a conductive varnish or by moistening.
When preparing a non-conductive object made of humidity absorbing material, the surface of the object can be made conductive by heating the object to a temperature at which humidity absorbed by the material is produced on the surface thereby forming a conductive layer.
Third, application of the powder, preferably by spraying while the powder particles are electrostatically charged and in such a way that they achieve a good distribution on the object.
Fourth, heating to make the powder particles melt to a leveled layer and adhere to the surfaces of the object.
Fifth, exposing the object to radiation, preferably ultraviolet radiation, thus initiating the curing process.
From this it is evident that the method may be carried out when coating objects made of non-conductive materials.
A polarity difference between the powder and object may, however, be valuable in order to get the powder distributed over all surfaces of the object, especially if it has a complicated configuration. Thus, the method does not require, but does not exclude, any form of charging or neutralization of the object. For instance, when objects made of non-conductive material are to be coated, by applying any of the methods herein disclosed, coating with a conductive varnish, or moistening. Moreover, an electrostatic charge is attained in certain materials when heated, a condition which may be utilized in certain circumstances.
The present invention also comprises a plant for carrying out the method.
In accordance with one embodiment of the present invention there is described a plant for powder coating the surface of objects with polymeric powder having a melting temperature less than about 100xc2x0 C., the polymeric powder including a polymer curable under the influence of electromagnetic radiation, the plant comprising conveying means for transporting the objects from a first end of the plant to a second end of the plant, the plant divided into chambers by a plurality of walls having openings for the passage of the objects; a first chamber at the first end of the plant including means for preparing the objects to retain the polymeric powder charged with static electricity in a subsequent spraying operation; a second chamber including spraying means for charging the polymeric powder with static electricity and for spraying by compressed air the polymeric powder onto the objects prepared in the first chamber, the second chamber further including means for controlling the temperature in the second chamber during the spraying from about 10xc2x0 C. to 40xc2x0 C. and for controlling the humidity of the air in the second chamber from about 20 to 80 percent relative humidity; a third chamber including infrared means for exposing the objects to infrared radiation and heating means for blowing air towards the objects thereby melting the powder retained on the objects, and means for controlling the heating means and the infrared means whereby the surface of the objects are maintained at a temperature no greater than about 100xc2x0 C.; an airlock before and after the third chamber to prevent heated air escaping from the third chamber and entering an adjacent chamber; and a fourth chamber including means for exposing the objects to electromagnetic radiation to initiate the curing of the polymeric powder, means for blowing air towards the objects, and means for controlling the temperature of the air to be blown from about 5xc2x0 C. to 40xc2x0 C.
In accordance with another embodiment of the present invention there is described a plant for powder coating the surface of heat sensitive and humidity absorbing objects with polymeric powder having a melting temperature less than about 100xc2x0 C., the polymeric powder including a polymer curable under the influence of electromagnetic radiation, the plant comprising an enclosure in the form of a tunnel having a first end and a second end; conveying means for transporting the objects to be coated from the first end of the tunnel to the second end of the tunnel at a transport velocity, the tunnel divided in compartments by means of a plurality of walls having openings to allow the passage of the objects conveyed by the conveying means; a first chamber at the first end of the tunnel for preparing the objects for retaining the polymeric powder in a subsequent spraying operation, the first chamber including means for exposing the objects to infrared radiation to heat the objects thereby producing moisture from humidity absorbed by the objects as condensate on the surface of the objects, the transport velocity of the conveying means being adapted so that the heating effect will prevent the condensate from being substantially vaporized; a second chamber including means for charging the polymeric powder with static electricity, means for spraying the polymeric powder onto the objects by compressed air, and means for grounding the objects having condensate coated surfaces; means for controlling the temperature within the second chamber during the spraying to about 10xc2x0 C. to 40xc2x0 C. and the humidity of the air within the second chamber to about 20 to 80 percent relative humidity; a third chamber including infrared means for exposing the objects to infrared radiation and heating means for blowing air against the objects thereby melting the powder retained on the objects, and means for controlling the heating means and the infrared means whereby the surface of the objects are maintained at a temperature no greater than about 100xc2x0 C.; an airlock before and after the third chamber to prevent heated air escaping from the third chamber and entering an adjacent chamber; and a fourth chamber including means for exposing the objects to electromagnetic radiation adapted to initiate the curing of the polymer, means provided for blowing air against the surfaces of the objects exposed to the electromagnetic radiation, and means for controlling the temperature of the air blown from about 5xc2x0 C. to 40xc2x0 C.
In accordance with another embodiment of the present invention there is described a plant for powder coating the surfaces of objects with polymeric powder having a melting temperature below about 100xc2x0 C., the polymeric powder includes a polymer curable under the influence of electromagnetic radiation, the plant comprising a first station including means for preparing the objects to retain the polymeric powder; a second station including spraying means for spraying the polymeric powder onto the objects prepared to retain the polymeric powder to provide a coating layer of polymeric powder; a third station including tempering means for heating the objects to the melting temperature of the polymeric powder to melt the coating layer of polymeric powder on the objects, the tempering means comprising means for directing a stream of heated air towards the objects and means for generating infrared radiation directed towards the surfaces of the objects; and a fourth station including means for emitting electromagnetic radiation to cure the coating layer of the polymeric powder.
In accordance with another embodiment of the present invention there is described a plant for powder coating the surfaces of heat sensitive objects with a polymeric particulate powder having a low melting temperature and a predetermined flow temperature, the polymeric powder including initiators adapted to initiate curing of the polymeric powder with ultra-violet radiation, the plant comprising a tunnel including conveying means for conveying the objects for polymeric powder coating through the tunnel, the tunnel including a first station including means for exposing the objects to infrared radiation; a second station including spraying means for spraying the polymeric powder onto the surfaces of the objects conveyed through the second station wherein the particles of the polymeric powder are subjected to electrostatic forces to provide a coating layer of melted polymeric powder on the surface of the objects; a third station operative for tempering the coating layer of melted polymeric powder on the surface of the objects comprising heating means for directing a stream of heated air towards the objects and means for exposing the objects to infrared radiation; a fourth station operative for curing the objects with ultraviolet radiation; and temperature control means for adapting the temperature of the airstream in the third station so that the coating layer of melted polymeric powder sprayed onto and retained on the surfaces of the objects in the second station reaches its melting temperature but not its flowing temperature.
In accordance with another embodiment of the present invention there is described a plant for powder coating the surfaces of objects with polymeric powder having a melting temperature less than about 100xc2x0 C., the polymeric powder including a polymer curable under the influence of electromagnetic radiation, the plant comprising a first chamber in which the objects to be coated are prepared to retain the polymeric powder charged with static electricity; a second chamber in which the objects are sprayed with the polymeric powder charged with static electricity in an atmosphere with the temperature in the range of about 10-40xc2x0 C. and about 20-80 percent relative humidity; a third chamber in which the objects are heated by exposure to infrared radiation to a surface temperature of no greater than about 100xc2x0 C. thereby melting the powder retained on the objects and by heated air directed towards the objects, the heated air and the infrared radiation having a temperature adapted to maintain the surface of the objects at no greater than about 100xc2x0 C.; and a fourth chamber in which the objects are radiated by electromagnetic radiation for initiating the curing of the heated polymeric powder and simultaneously cooled by flowing air having a temperature of about 5xc2x0 C. to 40xc2x0 C. for maintaining the objects at a temperature below the melting temperature of the powder thereby compensating for the heating of the objects from the electromagnetic radiation.
In accordance with another embodiment of the present invention there is described a plant for powder coating the surfaces of objects with polymeric powder having a melting temperature less than about 100xc2x0 C., the polymeric powder including a polymer curable under the influence of electromagnetic radiation, the plant comprising a first chamber for preparing the objects to be coated for retaining the polymeric powder; a second chamber for spraying the objects with the polymeric powder having an electrostatic charge in a first conditioned atmosphere; a third chamber for melting the polymer powder retained on the objects by heating the polymer powder by exposure to infrared radiation and by simultaneously directing heated air towards the objects, the heated air and the infrared radiation having a temperature adapted to maintain the surface of the objects less than about 100xc2x0 C.; and a fourth chamber for radiating the objects with electromagnetic radiation for initiating the curing of the heated polymeric powder and simultaneous cooling the objects by flowing conditioned air over the objects, whereby the objects are maintained at a temperature below the melting temperature of the polymer powder thereby compensating for the heating of the objects from the electromagnetic radiation.
In accordance with another embodiment of the present invention there is described a plant for powder coating the surface of heat sensitive objects polymer curable under the influence of electromagnetic radiation, the plant comprising a first chamber for storing the objects in an atmosphere having a predetermined relative humidity whereby the objects absorb moisture from the atmosphere, and heating means for pre-heating the objects by infrared radiation until the temperature at the surface of the objects has reached about 40xc2x0 C. to 85xc2x0 C. while retaining moisture thereon; a second chamber for spraying the objects in an atmosphere having an air temperature of about 10xc2x0 C. to 40xc2x0 C. with the polymeric powder having an electrostatic charge, and means for grounding the objects so that the polymeric powder will be retained at the surface of the objects by means of electrostatic forces, the grounded objects having electrical conducting properties resulting from the moisture produced on the surface of the objects in the first chamber; a third chamber for heating the objects to a temperature at which the polymeric powder melts by simultaneously using infrared radiation and means for blowing air having a predetermined temperature to maintain the surface temperature of the objects at less than about 100xc2x0 C.; and a fourth chamber for curing the polymeric powder by exposing the surface of the objects to electromagnetic radiation while simultaneously blowing air against the surface of the objects, whereby the temperature of the polymeric powder during the curing is maintained at a temperature not substantially exceeding the melting temperature of the polymeric powder.